An embryonic stem cell (hereinafter also referred to as ES cell) is an undifferentiated cell having the pluripotency and the ability of self-replication. In addition, it is suggested that the ES cell has the ability to repair damaged tissues. Therefore, intensive studies on ES cells have been conducted, since the ES cells are useful in the screening of therapeutic substances for various diseases, the field of regenerative medicine, and the like.
Currently, mouse-derived ES cells have been widely used in, for instance, the production of an engineered mouse in which a specific gene has been altered by gene targeting, and the like. However, in the case of the application of mouse-derived ES cells on the models of human diseases, the expected effects may not always be obtained for the following reasons: a) there are some genes expressed at different stages between mouse embryos and human embryos; b) the structures or functions of the extraembryonic tissues such as the placentas are different; c) the structures of the embryonic tissues of the early-embryos nidated are different.
On the other hand, monkey-derived embryonic stem cells are suitable for use for the studies of human diseases, because they are more closely related to human, as compared to mouse-derived ES cells.
Although approximately 200 kinds of monkeys have been conventionally known around the world, the types of monkeys used for the routine experiments are limited in the current situations. Higher primates can be roughly divided into the following two groups:
(1) New World Primates
The common marmoset (Callithrix jacchus) has been widely known, and used as an experimental primate. The development of the New World primate is basically similar to that of the Old World primate, with some differences regarding the structures of the embryos or the placentas.
(2) Old World Primates
The Old World primate is a primate extremely closely related to human. The rhesus monkey (Macaca mulatta) and the cynomolgus monkey (Macaca fascicularis) are known. The Japanese monkey (Macaca fuscata) belongs to the same genus (the genus Macaca) as Macaca fascicularis. The development of the Old World primate is extremely similar to that of human.
Currently, as monkey-derived ES cells, the common marmoset ES cells [Thomson, J. A. et al., Biol. Reprod. 55, 254–259 (1996)], and the rhesus ES cells [Thomson, J. A. et al., Proc. Natl. Acad. Sci. U.S.A. 92, 7844–7848 (1995)] have been established. However, as described above, Callithrix jacchus belongs to the New World primates which are not genealogically derived from humans, with some differences in the structures of the embryos or the placentas. Further, since Callithrix jacchus is small, the manipulations in various experiments are not easy in the current situations, and there are little background data. On the other hand, Macaca mulatto is rarely used as an experimental animal in Japan and Europe. In addition, the reproduction is seasonal, so that the regular ovulation does not necessarily occur throughout the year. Furthermore, there are defects in preparation of the common marmoset ES cells and the rhesus ES cells that the collection of ova is time-consuming and inefficient.
Although human ES cells also have been developed, the use thereof may be limited from the ethical viewpoint.